This invention relates to a process for increasing the thermal stability of a vinyl aromatic polymerization inhibitor by adding an inhibitor such as N,Nxe2x80x2-di-2-butyl-N,Nxe2x80x2-dinitroso-1,4-diaminobenzene to a stabilizer compound such as N,Nxe2x80x2-di-2-butyl-1,4-diaminobenzene. The mixture of inhibitor and stabilizer can be used in a process for inhibiting the polymerization of vinyl aromatic compounds such as styrene during its distillation.
Styrene is one of several vinyl aromatic compounds which has considerable commercial utility. Styrene is polymerized into polystyrene, which is a clear, readily colored and easily fabricated plastic with many uses. The efficiency of the polymerization process is dependent on the purity of the monomer starting material. Since the processes for producing styrene, and other vinyl aromatic compounds, will contain various reaction products including benzene, toluene, etc., the mixture is distilled to separate these undesirable contaminants. Unfortunately, the temperatures required for distillation, typically 90xc2x0 C. to about 150xc2x0 C., leads to the polymerization of the vinyl aromatic compounds. In order to minimize or prevent polymerization of the vinyl aromatic monomer, it is common to add a polymerization inhibitor to the distillation mixture.
The art discloses a variety of compounds, which are claimed to inhibit polymerization. These include U.S. Pat. No. 4,050,993, which discloses the use of N,N-nitroso-methylaniline as a polymerization inhibitor. U.S. Pat. No. 3,988,212 which disclose the use of N-nitrosodiphenyl amine in combination with dinitro-o-cresol. U.S. Pat. No. 4,013,580, which discloses the use of N-nitroso aniline derivatives. U.S. Pat. No. 4,341,600, which discloses the use of a mixture of dinitro-p-cresol and N-nitroso-diphenyl amine. U.S. Pat. No. 4,654,451, which discloses mixtures of alkyl substituted p-nitroso phenols and p-nitroso phenol. U.S. Pat. No. 5,034,156, which discloses N-nitrosophenyl-hydroxylamine plus hydroquinone monomethyl ether. U.S. Pat. No. 5,396,004, which discloses a phenylenediamine compound plus a hydroxyalkylhydroxyl-amine compound. U.S. Pat. No. 5,254,760, which discloses 1-oxyl-2,2,6,6-tetramethylpiperidine plus an aromatic nitro compound. U.S. Pat. No. 4,929,778, which discloses a phenylenediamine compound plus a hindered phenol compound. U.S. Pat. No. 5,312,952, which discloses the use of the reaction product of a C9-C20 alkyl phenol with sulfuric and nitric acid and optionally an aryl or alkyl-substituted phenylenediamine. WO9503263, which discloses 3,5-di-tert-butyl-4-hydroxy-N,N-dimethyl benzyl amine. EP-697386-A2, which discloses 4-acetylamino-2,2,6,6-tetramethyl piperidine N-oxyl in combination with 4-nitroso phenol. JP0701910-A which discloses phosphite compounds, nitrosoamine compounds or phenol compounds. JP05310815-A, which discloses the ammonium salt of N-nitrosophenyl hydroxylamine. JP03149205-A, which discloses nitrosophenols plus dicyclohexylammonium nitrate. J01226858-A, which discloses the use of substituted nitrosobenzene. U.S. Pat. No. 4,967,027, which discloses p-nitroso phenol plus p-t-butyl catechol. J59029624-A, which discloses the use of N-nitroso compound, e.g., N-nitroso-diphenylamine and a catechol, e.g., p-t-butylcatechol. Finally, U.S. Pat. No. 5,869,717, discloses the use of N-nitroso derivates of unsubstituted or dialkyl substituted phenylenediamine.
Applicants have now found that the thermal stability of the mono or di-nitroso compounds described in U.S. Pat. No. 5,869,717 can be increased by adding to the nitroso containing compound a stabilizer such as an unsubstituted or substituted phenylenediamine (diaminobenzene) or a substituted phenolic compound. The importance of this discovery is that it extends the temperature range over which the inhibitor can be used. This in turn means that the combination of inhibitor and stabilizer is able to withstand higher temperature disruptions or swings.
As stated, this invention relates to a process for increasing the thermal stability of various polymerization inhibitors and to a process for inhibiting the polymerization of a vinyl aromatic compound during its distillation. One embodiment of the invention is a process for increasing the thermal stability of a polymerization inhibitor selected from the group consisting of: 
and mixtures thereof, where R and R1 are each independently hydrogen, cyclohexyl, phenyl and an alkyl group having from 1 to 18 carbon atoms, the process comprising adding to the organic compound an effective amount of a stabilizer selected from the group consisting of: 
and mixtures thereof, where R2 is selected from the group consisting of hydrogen, phenyl, cyclohexyl and an alkyl group having from 1 to 18 carbon atoms, and R3 is selected from the group consisting of hydrogen, an alkyl group having from 1 to 18 carbon atoms, cyclohexyl, and an aromatic group having from 6 to 10 carbon atoms.
Another embodiment of the invention is a process for inhibiting the polymerization of a vinyl aromatic compound during the distillation of the vinyl aromatic compound comprising adding to the compound an effective amount of a mixture of at least one polymerization inhibitor and at least one stabilizer, where the inhibitor is selected from the group consisting of: 
and mixtures thereof, where R and R1 are each independently hydrogen, cyclohexyl, phenyl, or an alkyl group having from 1 to 18 carbon atoms, and the stabilizer is selected from the group consisting of: 
and mixtures thereof, where R2 is selected from the group consisting of hydrogen, cyclohexyl, phenyl, and an alkyl group having from 1 to 18 carbon atoms, and R3 is selected from the group consisting of hydrogen, an alkyl group having from 1 to 18 carbon atoms, cyclohexyl, and an aromatic group having by from 6 to 10 carbon atoms.
These and other objects and embodiments will become more evident after a detailed description of the invention.
The present invention relates to a process for increasing the thermal stability of a polymerization inhibitor and to a process for inhibiting the polymerization of a vinyl aromatic compound during its distillation. The polymerization inhibitors are selected from the group consisting of: 
The R and R1 groups can each independently be hydrogen or an alkyl group having from 1 to 18 carbon atoms. Illustrative of the alkyl groups which can be used are methyl, ethyl, propyl, butyl, pentyl, octyl, decyl, sec-butyl, isopropyl and isopentyl. Included in structures I and II above are the o, m and p isomers. Preferred compounds encompassed within structures I and II are the following compounds having structure IA and IIA. 
These nitroso compounds are prepared by reacting nitrous acid with an amine according to the procedure set forth in U.S. Pat. No. 5,869,717 which is incorporated by reference. The reaction of nitrous acid with an amine is well known in the art and is included here for completeness. Generally, the amine is reacted with nitrous acid at a temperature of about xe2x88x9210xc2x0 C. to about 120xc2x0 C. The nitrous acid can be added or it can be generated in situ by adding sodium nitrite and hydrochloric acid. Further, in the case of a diamine, the amount of nitrous acid added will determine whether the mono or dinitroso compounds are obtained. However, even if a 1:1 stoichiometric amount of nitrous acid:amine is added, a mixture of the mono and dinitroso compounds may still be obtained.
It has been found that the thermal stability of these polymerization inhibitors can be increased by adding to the inhibitor an effective amount of a stabilizer selected from the group consisting of: 
and mixtures thereof and R2 is selected from the group consisting of hydrogen, cyclohexyl, phenyl, and an alkyl group having from 1 to 18 carbon atoms, and R3 is selected from the group consisting of hydrogen, an alkyl group having from 1 to 18 carbon atoms, cyclohexyl and an aromatic group having from 6 to 10 carbon atoms. Preferred stabilizers are those represented by structure (IV) and an especially preferred stabilizer has structure (IVA) 
Of the various polymerization inhibitors, preferred ones are those having structure (II) and an especially preferred inhibitor is that having structure (IIA) an N,Nxe2x80x2-dinitroso-N,Nxe2x80x2-di-2-butyl-p-phenylenediamine. The amount of stabilizer necessary to increase the decomposition temperature is at least 0.01 wt. %, but can vary from about 0.01 to about 1 wt. % of the inhibitor.
Without wishing to be bound by any one particular theory, it appears that compounds such as (IVA) inhibit thermal decomposition by the following mechanism. It is first assumed that thermal decomposition of the nitroso compounds is triggered by cleavage of the nitroso group giving nitrous oxide (as evidenced by the evolution of a brown gas upon exposure to air). The nitrous oxide then catalyzes the further decomposition of the nitrosated inhibitor compound. One possible route for this further decomposition is the nitrous oxide induced cleavage of another nitrous oxide (autocatalysis).
Nitrous oxide is known to react with any primary or secondary amines such as phenylenediamines. Thus if a phenylenediamine is present when nitrous oxide is formed, then the nitrous oxide will react with the amine and not be available to induce further cleavage of the nitroso compounds. However, phenylenediamines are not the only compounds with which the nitrous oxide can react. The phenolic compounds (structures V and VI) mentioned above can also react with the nitrous oxide.
The mixture of inhibitor and stabilizer is used to prevent the polymerization of vinyl aromatic compounds during their distillation. Vinyl aromatic compounds to which this mixture can be added include but are not limited to styrene, alpha-methyl styrene, divinylbenzene, vinyl toluene, vinyl naphthalene and polyvinylbenzene. An effective amount of the mixture is that amount which will give from about 1 to about 10,000 ppm of the inhibitor.
Having added the effective amount of inhibitor plus stabilizer to the feed containing the vinyl aromatic compound, the feed is distilled at a temperature of about 65xc2x0 C. to about 150xc2x0 C. in standard distillation column. The distillation temperature can be controlled by adjusting the pressure in the column from about 0 kPa (0 psi) to about 165 Pa (24 psi) absolute. Further, the inhibitor and stabilizer mixture may be added to the vinyl aromatic compound in any convenient manner. Usually the mixture is added in liquid form by periodically or continuously adding the required amount to the inlet distillation feed. In this respect, if the inhibitor and/or stabilizer are solids, they are dissolved in an appropriate solvent. Illustrative of the solvents which can be used are the vinyl monomer itself, toluene, benzene, ethylbenzene, xylene, etc., and mixtures thereof. The preferred solvent is ethylbenzene which allows for easy separation from the purified vinyl aromatic compound.